Strategies to enhance microalgae anaerobic digestion in wastewater treatment systems : pretreatments and co-digestion

Premi extraordinari doctorat UPC curs 2017-2018. Àmbit d’Enginyeria Civil i AmbientalMicroalgae-based wastewater treatment systems are promising solutions to shift the paradigm from wastewater treatment to energy and resources recovery. In these systems, microalgae assimilate nutrients and produce oxygen which is used by bacteria to biodegrade organic matter improving water quality. Moreover, microalgae biomass can be harvested and reused to produce biofuels among other bioproducts. In this context, anaerobic digestion (AD) is one of the most consolidated and well-known technologies to convert organic waste generated in a wastewater treatment plant into bioenergy. However, microalgae AD is generally limited by their resistant cell wall, which lead to low methane potential (degradation extent) and conversion rate (degradation speed). Also, microalgae have high protein content, which can lead to ammonia nitrogen inhibition during the anaerobic digestion process. This PhD thesis aims to overcome these drawbacks and improve the technology by combining the use of pretreatments and the co-digestion. While pretreatments act disrupting or weakening the structure of microalgae cell wall, allowing the intracellular content to become more bioavailable, anaerobic co-digestion (i.e. the simultaneous digestion with two or more substrates) can contribute to improve microalgae AD performance by increasing methane potential, diluting inhibitory compounds or getting synergies between substrates (nutrients composition, rheology, etc.) in addition to the economic advantages derived from treating several wastes in a single facility. Firstly, co-digestion of harvested microalgae from high rate algal ponds (HRAP) used as secondary treatment for urban wastewater and primary sludge, which is produced in the same treatment process, is been investigated. Results have shown that the most suitable option to anaerobically digest microalgae from HRAPs would be the co-digestion with primary sludge at a 20-day hydraulic retention time (HRT), that leads to higher methane production (between 63% and 2.3-fold increase). The energy assessments conducted according to these results have revealed that microalgae co-digestion with primary sludge is a key technology for energy recovery in HRAPs, since the energy produced is up to 4-fold the energy consumed during the AD. Finally, potential reuse of microalgae digestates in agriculture has been investigated (including their co-digestion with primary sludge). all microalgae digestates have presented suitable properties for agricultural soils amendment, although digestate from co-digestion has presented the least phytotoxicity. Besides, co-digestion with storable agricultural wastes (i.e. wheat straw) is been evaluated. As it happens to microalgae, wheat straw AD is limited by hydrolysis step due to its lignocellulosic structure. Thus, their co-digestion with microalgae is also being investigated after a simultaneous thermo-alkaline pretreatment to both substrates. Results have shown that wheat straw co-digestion (50% VS) at 20-day HRT has increased microalgae methane yield by 77% as compared to microalgae mono-digestion (from 0.12 L CH4/g VS to 0.21 L CH4/gVS). On the other way around, pretreatment has only increased the methane yield by 15% as compared to untreated substrates co- digestion (0.24 L CH4/g VS). Thus, the co-digestion of microalgae and wheat straw is successful even without the pretreatment. Finally, when microalgae are used as tertiary treatment, waste activated sludge (WAS) results in abundant and available co-substrate. As a novelty, in this PhD thesis, microalgae and WAS co-digestion is investigated after applying a simultaneous autohydrolysis pretreatment at 55 °C to improve microalgae biodegradability by promoting inherent enzymes release from WAS. However, results have shown that WAS enzymes have not been effective at disrupting microalgae cell wall. Anyway, WAS co-digestion (80% VS) after pretreatment has increased microalgae mono-digestion methane yield up to 130%.Els sistemes de tractament d'aigües residuals amb microalgues són solucions tecnològiques que permeten canviar el paradigma del tractament d'aigües residuals a la recuperació d'energia i recursos. En aquests sistemes, les microalgues assimilen nutrients i produeixen oxigen que utilitzen els bacteris per a la biodegradació de matèria orgànica, millorant així la qualitat de l'aigua. A més, la biomassa de microalgues es pot recol·lectar i reutilitzar per produir biocombustibles. En aquest context, la digestió anaeròbia és una de les tecnologies més establertes que permeten convertir els residus orgànics generats en una depuradora en bioenergia. No obstant això, la digestió anaeròbia de microalgues està generalment limitada per la seva resistent paret cel·lular, i per aquest motiu presenten un baix potencial de metà i una Baixa taxa de degradació (velocitat de degradació). A més, les microalgues tenen un elevat contingut en proteïnes, fet que pot conduir a la inhibició per amoníac durant el procés de digestió anaeròbia. Aquesta tesi doctoral pretén millorar la tecnologia de la digestió anaeròbica combinant l’aplicació de pretractaments amb la co-digestió. Mentre que els pretractaments actuen per alterar o debilitar l'estructura de la paret cel·lular de les microalgues, permetent que el contingut intracel·lular sigui biodisponible, la co-digestió (és a dir, la digestió simultània amb dos o més substrats) pot contribuir a millorar el rendiment de la digestió de les microalgues augmentant el potencial de metà, diluint compostos inhibidors o fomentant sinergies entre substrats (composició de nutrients, reologia, etc.), a més dels avantatges econòmics derivats del tractament de diversos residus en una única instal·lació. En primer lloc, s'ha investigat la co-digestió de les microalgues procedents de llacunes d’alta càrrega (LLAC), utilitzades com a tractament secundari per a aigües residuals urbanes, i fangs primaris, que es produeixen en el mateix procés de tractament. Els resultats obtinguts indiquen que l'opció més adequada per digerir microalgues és amb la codigestió amb fang primari en un temps de retenció hidràulica (TRH) de 20 dies. Els balanços energètics duts a terme d'acord amb aquests resultats han mostrat que l'energia produïda és fins a 4 vegades l'energia consumida durant la digestió anaeròbica. Finalment, s'ha investigat la possible reutilització dels efluents de la digestió de microalgues en l'agricultura (inclosa la seva co-digestió amb fang primari). Tots els digestats de microalgues han presentat propietats adequades per se utilitzats com esmena de sòls agrícoles, tot i que l’efluent procedent de la codigestió ha presentat la menor fitotoxicitat. Complementàriament, s'ha avaluat la codigestió amb residus agrícoles que puguin ser emmagatzemables (palla de blat). Com passa amb les microalgues, la digestió anaeròbia de palla de blat està limitada per hidròlisi a causa de la seva estructura lignocel·lulosica. Per tant, la seva codigestió ambles microalgues també s'ha investigant després d'un pretractament simultani a tots dos substrats (termoalcalí). Quan les microalgues s'han co-digerit amb palla de blat, el rendiment del metà ha augmentat des de 0,12 L CH4 / g VS fins a 0,21 L CH4 / gVS (augment del 77%), mentre que el pretractament només ha augmentat el rendiment del metà en un 15% en comparació amb la codigestió dels substrats no tractats (0,24 L CH4 / g VS). Per últim, s’ha investigat la codigestió de microalgues i fangs biològics després d'aplicar un pretractament simultani a ambdós substrats d'autohidròlisi (55 °C). L’objectiu d’aquesta estratègia és millorar la biodegradabilitat de les microalgues per mitjà de l'alliberament d'enzims inherents als fangs. Tot i que en els assajos s’ha vist que els enzims alliberats pels fangs no han estat eficaços degradant la paret cel·lular de les microalgues, la codigestió amb els fangs biològics després del pretractament ha permès permet augmentar la producció de metà de les microalgues fins a un 130%.Award-winningPostprint (published version

Enhancement of microalgae anaerobic digestion by thermo-alkaline pretreatment with lime (CaO)

The aim of this study was to evaluate for the first time the effect of a thermo-alkaline pretreatment with lime (CaO) on microalgae anaerobic digestion. The pretreatment was carried out by adding different CaO doses (4 and 10%) at different temperatures (room temperature (25 °C), 55 and 72 °C). The exposure time was 4 days for pretreatments at 25 °C, and 24 h for pretreatments at 55 and 72 °C. Following, a biochemical methane potential test was conducted with pretreated and untreated microalgae. According to the results, the pretreatment enhanced proteins solubilisation by 32.4% and carbohydrates solubilisation by 31.4% with the highest lime dose and temperature (10% CaO and 72 °C). Furthermore, anaerobic digestion kinetics were improved in all cases (from 0.08 to 0.14 day- 1 for untreated and pretreated microalgae, respectively). The maximum biochemical methane potential increase (25%) was achieved with 10% CaO at 72 °C, in accordance with the highest biomass solubilisation. Thus, lime pretreatment appears as a potential strategy to improve microalgae anaerobic digestion.Peer ReviewedPostprint (author's final draft

Anaerobic co-digestion of microalgal biomass and wheat straw with and without thermo-alkaline pretreatment

This study aimed at analyzing the anaerobic co-digestion of microalgal biomass grown in wastewater and wheat straw. To this end, Biochemical Methane Potential (BMP) tests were carried out testing different substrate proportions (20–80, 50–50 and 80–20%, on a volatile solid basis). In order to improve their biodegradability, the co-digestion of both substrates was also evaluated after applying a thermo-alkaline pretreatment (10% CaO at 75 °C for 24 h). The highest synergies in degradation rates were observed by adding at least 50% of wheat straw. Therefore, the co-digestion of 50% microalgae – 50% wheat straw was investigated in mesophilic lab-scale reactors. The results showed that the methane yield was increased by 77% with the co-digestion as compared to microalgae mono-digestion, while the pretreatment only increased the methane yield by 15% compared to the untreated mixture. Thus, the anaerobic co-digestion of microalgae and wheat straw was successful even without applying a thermo-alkaline pretreatment.Peer ReviewedPostprint (author's final draft

Mejora de la digestión anaerobia de microalgas mediante codigestión y pretratamientos

Los sistemas de tratamiento de aguas residuales con microalgas combinan el tratamiento de aguas residuales y la producción de bioenergía. Algunos estudios han demostrado la viabilidad de producir biogás a partir de microalgas cultivadas en estaciones depuradoras de aguas residuales (EDAR). Sin embargo, existen algunas limitaciones, como las características de la pared celular de las microalgas que limita su biodegradabilidad, su alto contenido en nitrógeno y que su producción sigue un patrón estacional. Para superar estas limitaciones, el proceso de digestión anaerobia se podría mejorar mediante la aplicación de pretratamientos para incrementar la biodegradabilidad de las microalgas y/o la codigestión con otros sustratos ricos en carbono, para equilibrar el ratio C/N y lograr altas velocidades de carga orgánica (VCO) a lo largo del año. En el presente trabajo se evaluaron diferentes estrategias para mejorar la digestión anaerobia de microalgas, basadas en su codigestión con residuos producidos en la propia EDAR (lodo primario y grasas) y con subproductos agrícolas (paja de trigo). Además, se aplicaron diferentes pretratamientos según el tipo de sustrato a codigerir, seleccionando los que requieren un menor aporte de energía y que conllevan el menor impacto ambiental. Según los resultados obtenidos, la codigestión de microalgas pretratadas térmicamente con lodos primarios es la alternativa más adecuada para mejorar la digestión microalgas. La codigestión de estos sustratos en reactores en continuo mostró un incremento de la producción de biogás del 155% comparado con la producción de las microalgas sin pretratar ni codigerir. Además, la codigestión con grasa de EDAR permitiría incrementar aún más la producción de metano (entre un 25-40% añadiendo solamente un 10-20% de sólidos volátiles en forma de grasa). Finalmente, la codigestión de microalgas con paja de trigo también permite incrementar la producción de metano, con y sin pretratamiento térmico-alcalino (>77%). Su principal ventaja es que la paja de trigo es un co-sustrato almacenable, que puede proporcionar carbono y aumentar las VCO durante las estaciones con baja producción de microalgas.Postprint (published version

Strategies to optimize microalgae conversion to biogas: co-digestion, pretreatment and hydraulic retention time

This study aims at optimizing the anaerobic digestion (AD) of biomass in microalgal-based wastewater treatment systems. It comprises the co-digestion of microalgae with primary sludge, the thermal pretreatment (75 ◦C for 10 h) of microalgae and the role of the hydraulic retention time (HRT) in anaerobic digesters. Initially, a batch test comparing different microalgae (untreated and pretreated) and primary sludge proportions showed how the co-digestion improved the AD kinetics. The highest methane yield was observed by adding 75% of primary sludge to pretreated microalgae (339 mL CH4/g VS). This condition was then investigated in mesophilic lab-scale reactors. The average methane yield was 0.46 L CH4/g VS, which represented a 2.9-fold increase compared to pretreated microalgae mono-digestion. Conversely, microalgae showed a low methane yield despite the thermal pretreatment (0.16 L CH4/g VS). Indeed, microscopic analysis confirmed the presence of microalgae species with resistant cell walls (i.e., Stigioclonium sp. and diatoms). In order to improve their anaerobic biodegradability, the HRT was increased from 20 to 30 days, which led to a 50% methane yield increase. Overall, microalgae AD was substantially improved by the co-digestion with primary sludge, even without pretreatment, and increasing the HRT enhanced the AD of microalgae with resistant cell walls

Assessing the agricultural reuse of the digestate from microalgae anaerobic digestion and co-digestion with sewage sludge

Microalgae anaerobic digestion produces biogas along with a digestate that may be reused in agriculture. However, the properties of this digestate for agricultural reuse have yet to be determined. The aim of this study was to characterise digestates from different microalgae anaerobic digestion processes (i.e. digestion of untreated microalgae, thermally pretreated microalgae and thermally pretreated microalgae in co-digestion with primary sludge). The main parameters evaluated were organic matter, macronutrients and heavy metals content, hygenisation, potential phytotoxicity and organic matter stabilisation. According to the results, all microalgae digestates presented suitable organic matter and macronutrients, especially organic and ammonium nitrogen, for agricultural soils amendment. However, the thermally pretreated microalgae digestate was the least stabilised digestate in comparison with untreated microalgae and co-digestion digestates. In vivo bioassays demonstrated that the digestates did not show residual phytotoxicity when properly diluted, being the co-digestion digestate the one which presented less phytotoxicity. Heavy metals contents resulted far below the threshold established by the European legislation on sludge spreading. Moreover, low presence of E. coli was observed in all digestates. Therefore, agricultural reuse of thermally pretreated microalgae and primary sludge co-digestate through irrigation emerges a suitable strategy to recycle nutrients from wastewater

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Unveiling microbial structures during raw microalgae digestion and co-digestion with primary sludge to produce biogas using semi-continuous AnMBR systems

[EN] Methane production from microalgae can be enhanced through anaerobic co-digestion with carbon-rich substrates and thus mitigate the inhibition risk associated with its low C:N ratio. Acclimated microbial communities for microalgae disruption can be used as a source of natural enzymes in bioenergy production. However, co-substrates with a certain microbial diversity such as primary sludge might shift the microbial structure. Substrates were generated in a Water Resource Recovery Facility (WRRF) and combined as follows: Scenedesmus or Chlorella digestion and microalgae co-digestion with primary sludge. The study was performed using two lab-scale Anaerobic Membrane Bioreactors (AnMBR). During three years, different feedstocks scenarios for methane production were evaluated with a special focus on the microbial diversity of the AnMBR. 57% of the population was shared between the different feedstock scenarios, revealing the importance of Anaerolineaceae members besides Smithella and Methanosaeta genera. The addition of primary sludge enhanced the microbial diversity of the system during both Chlorella and Scenedesmus co-digestion and promoted different microbial structures. Aceticlastic methanogen Methanosaeta was dominant in all the feedstock scenarios. A more remarkable role of syntrophic fatty acid degraders (Smithella, Syntrophobacteraceae) was observed during co-digestion when only microalgae were digested. However, no significant changes were observed in the microbial composition during anaerobic microalgae digestion when feeding only Chlorella or Scenedesmus. This is the first work revealing the composition of complex communities for semi-continuous bioenergy production from WRRF streams. The stability and maintenance of a microbial core over-time in semi-continuous AnMBRs is here shown supporting their future application in full-scale systems for raw microalgae digestion or codigestion.The Ministry of Economy and Competitiveness (MINECO) and the European Regional Development Fund (ERDF) are gratefully acknowledged for their support to this research work through CTM2011-28595-C02-02 and CTM2014-54980-C2-1-R projects. The authors are thankful to Ph.D. Silvia Greses and Ph.D. candidate Rebecca Serna-Garcia (Universitat de Valencia, Spain) for allowing the collection of digestate samples from their bioreactors and providing a brief data characterization of their performance. As well, authors thank the support of Maria Paches (IIAMA, Valencia, Spain) during phytoplankton monitoring in the photobioreactor plant. 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Strategies to enhance microalgae anaerobic digestion in wastewater treatment systems : pretreatments and co-digestion

Microalgae-based wastewater treatment systems are promising solutions to shift the paradigm from wastewater treatment to energy and resources recovery. In these systems, microalgae assimilate nutrients and produce oxygen which is used by bacteria to biodegrade organic matter improving water quality. Moreover, microalgae biomass can be harvested and reused to produce biofuels among other bioproducts. In this context, anaerobic digestion (AD) is one of the most consolidated and well-known technologies to convert organic waste generated in a wastewater treatment plant into bioenergy. However, microalgae AD is generally limited by their resistant cell wall, which lead to low methane potential (degradation extent) and conversion rate (degradation speed). Also, microalgae have high protein content, which can lead to ammonia nitrogen inhibition during the anaerobic digestion process. This PhD thesis aims to overcome these drawbacks and improve the technology by combining the use of pretreatments and the co-digestion. While pretreatments act disrupting or weakening the structure of microalgae cell wall, allowing the intracellular content to become more bioavailable, anaerobic co-digestion (i.e. the simultaneous digestion with two or more substrates) can contribute to improve microalgae AD performance by increasing methane potential, diluting inhibitory compounds or getting synergies between substrates (nutrients composition, rheology, etc.) in addition to the economic advantages derived from treating several wastes in a single facility. Firstly, co-digestion of harvested microalgae from high rate algal ponds (HRAP) used as secondary treatment for urban wastewater and primary sludge, which is produced in the same treatment process, is been investigated. Results have shown that the most suitable option to anaerobically digest microalgae from HRAPs would be the co-digestion with primary sludge at a 20-day hydraulic retention time (HRT), that leads to higher methane production (between 63% and 2.3-fold increase). The energy assessments conducted according to these results have revealed that microalgae co-digestion with primary sludge is a key technology for energy recovery in HRAPs, since the energy produced is up to 4-fold the energy consumed during the AD. Finally, potential reuse of microalgae digestates in agriculture has been investigated (including their co-digestion with primary sludge). all microalgae digestates have presented suitable properties for agricultural soils amendment, although digestate from co-digestion has presented the least phytotoxicity. Besides, co-digestion with storable agricultural wastes (i.e. wheat straw) is been evaluated. As it happens to microalgae, wheat straw AD is limited by hydrolysis step due to its lignocellulosic structure. Thus, their co-digestion with microalgae is also being investigated after a simultaneous thermo-alkaline pretreatment to both substrates. Results have shown that wheat straw co-digestion (50% VS) at 20-day HRT has increased microalgae methane yield by 77% as compared to microalgae mono-digestion (from 0.12 L CH4/g VS to 0.21 L CH4/gVS). On the other way around, pretreatment has only increased the methane yield by 15% as compared to untreated substrates co- digestion (0.24 L CH4/g VS). Thus, the co-digestion of microalgae and wheat straw is successful even without the pretreatment. Finally, when microalgae are used as tertiary treatment, waste activated sludge (WAS) results in abundant and available co-substrate. As a novelty, in this PhD thesis, microalgae and WAS co-digestion is investigated after applying a simultaneous autohydrolysis pretreatment at 55 °C to improve microalgae biodegradability by promoting inherent enzymes release from WAS. However, results have shown that WAS enzymes have not been effective at disrupting microalgae cell wall. Anyway, WAS co-digestion (80% VS) after pretreatment has increased microalgae mono-digestion methane yield up to 130%.Els sistemes de tractament d'aigües residuals amb microalgues són solucions tecnològiques que permeten canviar el paradigma del tractament d'aigües residuals a la recuperació d'energia i recursos. En aquests sistemes, les microalgues assimilen nutrients i produeixen oxigen que utilitzen els bacteris per a la biodegradació de matèria orgànica, millorant així la qualitat de l'aigua. A més, la biomassa de microalgues es pot recol·lectar i reutilitzar per produir biocombustibles. En aquest context, la digestió anaeròbia és una de les tecnologies més establertes que permeten convertir els residus orgànics generats en una depuradora en bioenergia. No obstant això, la digestió anaeròbia de microalgues està generalment limitada per la seva resistent paret cel·lular, i per aquest motiu presenten un baix potencial de metà i una Baixa taxa de degradació (velocitat de degradació). A més, les microalgues tenen un elevat contingut en proteïnes, fet que pot conduir a la inhibició per amoníac durant el procés de digestió anaeròbia. Aquesta tesi doctoral pretén millorar la tecnologia de la digestió anaeròbica combinant l’aplicació de pretractaments amb la co-digestió. Mentre que els pretractaments actuen per alterar o debilitar l'estructura de la paret cel·lular de les microalgues, permetent que el contingut intracel·lular sigui biodisponible, la co-digestió (és a dir, la digestió simultània amb dos o més substrats) pot contribuir a millorar el rendiment de la digestió de les microalgues augmentant el potencial de metà, diluint compostos inhibidors o fomentant sinergies entre substrats (composició de nutrients, reologia, etc.), a més dels avantatges econòmics derivats del tractament de diversos residus en una única instal·lació. En primer lloc, s'ha investigat la co-digestió de les microalgues procedents de llacunes d’alta càrrega (LLAC), utilitzades com a tractament secundari per a aigües residuals urbanes, i fangs primaris, que es produeixen en el mateix procés de tractament. Els resultats obtinguts indiquen que l'opció més adequada per digerir microalgues és amb la codigestió amb fang primari en un temps de retenció hidràulica (TRH) de 20 dies. Els balanços energètics duts a terme d'acord amb aquests resultats han mostrat que l'energia produïda és fins a 4 vegades l'energia consumida durant la digestió anaeròbica. Finalment, s'ha investigat la possible reutilització dels efluents de la digestió de microalgues en l'agricultura (inclosa la seva co-digestió amb fang primari). Tots els digestats de microalgues han presentat propietats adequades per se utilitzats com esmena de sòls agrícoles, tot i que l’efluent procedent de la codigestió ha presentat la menor fitotoxicitat. Complementàriament, s'ha avaluat la codigestió amb residus agrícoles que puguin ser emmagatzemables (palla de blat). Com passa amb les microalgues, la digestió anaeròbia de palla de blat està limitada per hidròlisi a causa de la seva estructura lignocel·lulosica. Per tant, la seva codigestió ambles microalgues també s'ha investigant després d'un pretractament simultani a tots dos substrats (termoalcalí). Quan les microalgues s'han co-digerit amb palla de blat, el rendiment del metà ha augmentat des de 0,12 L CH4 / g VS fins a 0,21 L CH4 / gVS (augment del 77%), mentre que el pretractament només ha augmentat el rendiment del metà en un 15% en comparació amb la codigestió dels substrats no tractats (0,24 L CH4 / g VS). Per últim, s’ha investigat la codigestió de microalgues i fangs biològics després d'aplicar un pretractament simultani a ambdós substrats d'autohidròlisi (55 °C). L’objectiu d’aquesta estratègia és millorar la biodegradabilitat de les microalgues per mitjà de l'alliberament d'enzims inherents als fangs. Tot i que en els assajos s’ha vist que els enzims alliberats pels fangs no han estat eficaços degradant la paret cel·lular de les microalgues, la codigestió amb els fangs biològics després del pretractament ha permès permet augmentar la producció de metà de les microalgues fins a un 130%